Seed size is a key determinant of evolutionary fitness in plants and is also an important agronomic trait during crop domestication (Orsi and Tanksley, 2009). Several studies suggest that seedlings of large-seeded plants are better able to tolerate many of the stresses encountered during seedling establishment, whereas small-seeded plants are considered to have superior colonization abilities because they produce large numbers of seeds (Westoby et al., 2002; Moles et al., 2005). At the same time, seed size is negatively associated with the number of seeds produced by a plant due to the limited resources of the mother plant (Harper et al., 1970). Scientific interest in seed size relates not only to its importance in plant fitness, but also to crop domestication. Crops domesticated for consumption of their seeds (e.g. rice and wheat) often produce seeds significantly larger than their wild ancestors (Fan et al., 2006; Song et al., 2007; Gegas et al., 2010).
A seed consists of three major components; the embryo, the endosperm and the seed coat, that originate from different cells of the ovule and possess different complements of maternal and paternal genomes. In angiosperms, seed development involves a double fertilization process in which one sperm nucleus fuses with the egg to produce the diploid embryo, while the other sperm nucleus fuses with two polar nuclei to form the triploid endosperm (Lopes and Larkins, 1993). The seed coat differentiates after fertilization from maternally derived integuments. The embryo is surrounded by the endosperm, which, in turn, is enclosed within the maternal seed coat. Therefore, the size of a seed is determined by the coordinated growth of maternal sporophytic and zygotic tissues.
The size of seeds is influenced by a variety of cellular processes. Seed size is known to be influenced by parent-of-origin effects. The cross between a diploid female parent and tetraploid male parent produces larger F1 seeds, whereas the reciprocal cross generates smaller F1 seeds, suggesting that maternal or paternal excess of genome has a dramatic effect on seed size (Scott et al., 1998). Similar to interploidy crosses, crosses between wild type and met1 mutant with hypomethylated genomes show that larger F1 seeds are generated when the maternal parent is met1, while smaller F1 seeds are produced when the paternal parent is met1 (Xiao et al., 2006), suggesting that parent-of-origin effects may involve DNA methylation. In addition, the size of seeds is affected by the maternal and/or zygotic tissues. Several factors that influence seed size by the zygotic tissues have been recently identified in Arabidopsis. haiku (iku) and miniseed3 (mini3) mutants form small seeds due to the reduced growth and early cellularization of the endosperm (Garcia et al., 2003; Luo et al., 2005). IKU1, IKU2 and MINI3 function in the same pathway to promote endosperm growth in Arabidopsis (Garcia et al., 2003; Luo et al., 2005; Wang et al., 2010). SHORT HYPOCOTYL UNDER BLUE1 (SHB1) associates with both MINI3 and IKU2 promoters in vivo and may act with other proteins that bind to MINI3 and IKU2 promoters to promote endosperm growth in the early phase of seed development (Zhou et al., 2009). Seed size is also influenced by maternal tissues. Several factors that act in maternal tissues to influence seed size have been isolated. Arabidopsis TRANSPARENT TESTA GLABRA 2 (TTG2) promotes seed growth by increasing cell expansion in the integuments (Garcia et al., 2005; Ohto et al., 2009). APETALA2 (AP2) may restrict seed growth by limiting cell expansion in the integuments (Jofuku et al., 2005; Ohto et al., 2005; Ohto et al., 2009). By contrast, AUXIN RESPONSE FACTOR 2 (ARF2) and the predicted ubiquitin receptor CYP78A61 limit seed size by restricting cell proliferation in the integuments (Schruff et al., 2006; Li et al., 2008). However, CYP78A5/KLU promotes seed growth by increasing cell proliferation in the integuments of ovules (Adamski et al., 2009). Therefore, the integument or seed coat plays a key role in maternal control of seed size. In addition, many quantitative trait loci (QTLs) for seed size have been mapped in Arabidopsis and crops (Alonso-Blanco et al., 1999; Li et al., 2004; Fan et al., 2006; Song et al., 2007; Shomura et al., 2008; Weng et al., 2008). Three grain size QTLs have been recently cloned in rice, including GS3, GW2 and qSW5/GW5 (Fan et al., 2006; Song et al., 2007; Shomura et al., 2008; Weng et al., 2008). However, it is not clear whether these three factors act in maternal and/or zygotic tissues in rice.
Despite the importance of seed size, relatively little is known about the genetic and molecular mechanisms that control seed size.
Identification of factors that control the final size of seeds will not only advance understanding of the mechanisms of size control in plants, but may also have substantial practical applications for example in improving crop yield.